The present invention relates to a method for seal testing capacitive sensors, arranged in a hermetically sealed enclosure, in which the processed sensors are arranged in the form of a wafer.
Acceleration sensors manufactured by surface micromechanical engineering frequently work according to the capacitive measuring principle. Such an acceleration sensor is shown in FIG. 4a. In the capacitive measuring principle, a moveable test mass, with electrode combs (so-called moving fingers) attached to the sides, is suspended on tiny silicon springs in such a manner that it is deflected by accelerations in the sensitive direction. In so doing, the capacitance between the fixed and the moveable electrodes is increased on one side of the test mass and decreased on the other side. The sensor fingers, which are electrically isolated from one another, thus form a differential capacitor with a capacitance of around 600 fF.
It is absolutely essential to protect the processed wafers from moisture, particles and other mechanical influences by means of a cover in the form of a hermetically sealing lid made of silicon or a similar material.
Known leak and seal testing methods for testing the hermeticity according to MIL or EN standards (for example the xe2x80x9cbubble testxe2x80x9d or xe2x80x9ccoarse leakxe2x80x9d according to MIL method 1014) cannot be used because of the extremely small volume of the sensor cavity. Other known methods of seal testing, either cannot be used at wafer level or can only be used with high expenditure on safety because of the use of radio isotopes, such as krypton 85, and therefore cannot be used for series production because the costs are too high.
The object of the invention is to provide a testing method, with which capacitive sensors can be tested for tightness cost-effectively, reliably and in an environmentally compatible manner.
According to the invention, there is a method for testing the tightness of capacitive sensors arranged in a hermetically sealed enclosure, in which the processed sensors are arranged in the form of a wafer, in which the already sawn wafer bearing the sensors is immersed in a test fluid under defined conditions, the capacitance of each sensor is then measured and compared with the capacitance of reference sensors.
The method according to the invention possesses the advantages that the test fluid, with defined dielectrical properties, penetrating into the cavity enables the finest leaks, up into the so-called coarse leak range (10xe2x88x923 to 10xe2x88x925 mbarxc2x7dm3xc2x7sxe2x88x921), to be detected by clearly measurable capacitance changes. A further advantage lies in the fact that the test method can be applied as an xe2x80x9cinline test methodxe2x80x9d, that is the seal test can be integrated into the normal wafer test in the form of 100% testing at wafer level without incurring additional testing time. Moreover, no negative effects (damage by oxidation, corrosion etc.) on the test piece are to be expected as a consequence of using the test fluids. Furthermore, no reaction is expected with the wafer clamped on foil.
The invention is particularly suitable for leak and sealing tests for micromechanically constructed and hermetically sealed capacitive sensors, in particular acceleration sensors.
Advantageous embodiments of the method according to claim 1 are stated in the subclaims.